The main function of EGR exchangers is to exchange heat between the exhaust gases and the coolant for the purpose of cooling gases.
Currently, EGR heat exchangers are used extensively in diesel applications to reduce emissions and also in petrol applications to reduce the consumption of fuel.
The current market trend is to reduce the size of engines and install EGR heat exchangers, not only in high pressure (HP) applications but also in low pressure (LP) applications. However, these two types of application have an impact on the design of EGR heat exchangers. Vehicle manufacturers are demanding EGR heat exchangers with improved outputs but at the same time the space available for installing an exchanger and its components is becoming smaller and increasingly difficult to incorporate.
Furthermore, in many applications the flow of coolant for cooling exhaust gases has a tendency to decrease despite the increase in output of the exchanger.
The current design of EGR exchangers on the market corresponds to a metal heat exchanger, generally made from stainless steel or aluminium.
There are basically two types of EGR heat exchangers: a first type consisting of a casing containing a bundle of parallel tubes for the passage of gases, the coolant circulating in the casing around the tubes; and the second type comprising a series of parallel plates which form the heat exchange surfaces, such that the exhaust gases and the coolant circulate between two plates in alternate layers and it can comprise fins to improve the exchange of heat.
In the case of heat exchangers comprising a bundle of tubes the junction between the tubes and the casing can differ. Generally, the tubes are fixed at their ends between two support plates connected to each end of the casing, the two support plates having a plurality of openings for the insertion of the respective tubes.
Said support plates are fixed in turn to means for connecting to the recirculation line which can consist of a V-shaped connector or even a peripheral connecting rim or flange, depending on the design of the recirculation line where the exchanger is assembled. The peripheral rim can either be mounted with a gas reservoir, so that the gas reservoir is an intermediate part between the casing and the rim, or can be mounted directly onto the casing.
In both types of EGR exchanger most of the components are made of metal and are therefore assembled by mechanical means, then oven soldered or arc welded to guarantee the required degree of sealing for this application.
A known type of exchanger comprises a bundle of tubes with a basically rectangular cross section distributed over two adjacent columns and a plurality of rows, the height of the tubes being less than their width. Said bundle of tubes is housed in a basically rectangular casing, with the gas inlet and gas outlet located at opposite ends of the casing.
This type of exchanger also comprises two pipes connected to the casing, for the inlet and outlet of coolant respectively. The coolant has to circulate around the tubes and in particular cool the support plate located at the gas inlet effectively because of the raised temperature of said plate. In this case, it is necessary to ensure good circulation of the coolant in the gas inlet area to avoid the formation of low flow areas which would imply a local increase in the temperature of the coolant by exchange with the inlet gases at high temperatures.
The distribution of coolant in the casing between the gas tubes depends on the dimensions of the casing and the position of the coolant pipes. In specific configurations there is a problem that boiling may occur which is associated with a poor distribution of coolant close to the support plate of the gas inlet. Thus, the more effective the distribution of coolant in the area adjacent to the support plate of the gas inlet the easier it is to control the problem of boiling caused by the raised temperature of the tubes in said area.
In a known configuration the coolant inlet pipe is connected to aside of the casing, close to the underside and the gas outlet, whereas the coolant outlet pipe is connected to the topside of the casing, in the centre and close to the gas inlet. This configuration thus enables a counter-current circulation of the coolant. In this case the coolant outlet pipe is located above the space which separates the two columns of tubes, said space between the tubes being relatively small which makes the outflow of the coolant more difficult.
It should be noted that when the exchanger is used with parallel circulation, i.e. when the coolant inlet pipe is arranged close to the gas inlet, said boiling problems also occur.
In another known configuration with counter-current circulation, the coolant inlet pipe is connected beneath the casing, close to the gas outlet, whereas the coolant outlet pipe is connected to a side of the casing close to the gas inlet. In this case, the coolant outlet pipe takes up several spaces between the rows of tubes, as the height of the tubes is less than their width. The surface passed over by the coolant is thus greater between the tubes towards the outlet.
Consequently, in this last configuration the problem of boiling is improved, on the one hand because the flow of coolant is greater in the outlet area and on the other hand because its distribution between the tubes is more uniform. However, this configuration is not achievable in some arrangements and sizes of the engine space where the orientation of the connecting sleeve to the coolant outlet pipe is not satisfactory.